The present invention is related to a dental amalgam, and in particular to an amalgamatable dental alloy powder for making an amalgam having a low initial mercury vapor release rate.
Amalgamatable dental alloys are typically silver based alloys in particulate form which can be mixed with mercury to produce an amalgam which is initially soft and pliable but which sets to a hard mass after a time. Thus, when fresh amalgam is placed in a tooth cavity it can be worked by the dentist to completely fill the cavity and to have an external configuration consistent with that of the remainder of the tooth.
It was known that an amalgamgtable alloy powder can be prepared from an all-in-one alloy having a desired composition or can be formed by mixing an amalgamatable silver-base alloy matrix powder and a non-amalgamatable alloy dispersant powder in a controlled ratio to meet the desired composition. Such alloy powders can be in the form of irregularly-shaped microgranules, flakes or filings, made by mechanical refining; or in the form of spherical particles derived from a microcasting of a molten mass of the alloy, such as by fluid atomization techniques or the like.
Two of the present inventors and their co-worker have studied the effect of particle configuration on the structure and properties of four dispersed types of Pd-containing amalgam alloy powder admixtures with the same composition, including a blend of spherical (atomized) matrix particles with spherical dispersant particles; a blend of spherical matrix particles with irregular (lathe-cut) dispersant particles; a blend of irregular matrix particles with spherical dispersant particles; and a blend of irregular matrix particles with irregular dispersant particles [K. I. Chen, C. P. Ju, and J. H. Chern Lin, entitled xe2x80x9cEffect of particle configuration on structure and properties of dispersed Pd-containing dental amalgamxe2x80x9d, Biomaterials 20 (1999) 1851-1866]. One of the properties investigated therein was an early stage mercury vapor release rate of amalgam, and the investigation showed that the early stage mercury vapor release rate of the amalgams comprising irregular dispersant particles were significantly lower than those of amalgams comprising spherical dispersant particles.
It is clearly that there is still a great interest in further reducing the early stage release rate of mercury vapor from the dental amalgam, which constitutes the major portion of the mercury vapor released from the dental amalgam during its preparation and when it is worn in vivo.
The present invention provides an amalgamatable dental alloy powder for making an amalgam having a low initial mercury vapor release rate having a composition comprising 50-80 wt % Ag; 10-30 wt % Cu, and 10-35 wt % Sn, and optionally less than 7 wt % of Pd, and having a particle size ranging from 1 to 100 microns with a majority thereof having a particle size less than 45 microns, which is prepared by a process comprising the following steps i) and ii), or steps I) and II):
i) subjecting a single-alloy powder having a particle size ranging from 1 to 55 microns with a majority thereof having a particle size less than 20 microns to a first heat treatment;
ii) subjecting the heat treated single-alloy powder to a pickling treatment;
I) subjecting a Agxe2x80x94Cuxe2x80x94Sn powder having a particle size ranging from 1 to 70 microns with a majority thereof having a particle size less than 30 microns to a second heat treatment, and subjecting a Agxe2x80x94Cuxe2x80x94Pd powder having a particle size ranging from 1 to 100 microns with a majority thereof having a particle size less than 45 microns to a third heat treatment;
II) subjecting the heat treated Agxe2x80x94Cuxe2x80x94Sn powder and the heat treated Agxe2x80x94Cuxe2x80x94Pd powder to a pickling treatment;
wherein said heat treatments comprise heating said powder at a temperature ranging from 100 to 600xc2x0 C. in an inert atmosphere for a period ranging from 0.5 hours to 8 days, preferably from 100 to 500xc2x0 C. and for a period ranging from 24 hours to 48 hours provided that said powder maintain a powder form or a loose agglomerate form after said heat treatment, whereby an amalgam made from said amalgamatable dental alloy powder has a reduced initial mercury vapor release rate.
The inventors of the present application also surprisingly found that the amalgamatable dental alloy powder of the present invention prepared from said single-alloy powder can have a significantly lower content of Sn, i.e. lower than 20 wt %, without jeopardizing the performance of the amalgams.
Preferably, the amalgamatable dental alloy powder of the present invention is free from Pd and is prepared by the process comprising steps i) and ii). Preferably, said composition of said amalgamatable dental alloy powder comprises 60-70 wt % Ag; 10-20 wt % Cu, and 10-30 wt % Sn, and more preferably 60 wt % Ag, 13 wt % Cu and 27 wt % Sn.
Alternatively, the amalgamatable dental alloy powder of the present invention comprises Pd and is prepared by the process comprising steps i) and ii). Preferably, said composition of said amalgamatable dental alloy powder comprises 10-20 wt % Sn, and more preferably comprises 60-70 wt % Ag; 10-20 wt % Cu, and 2-4 wt % Pd.
Preferably, said first heat treatment comprises heating said single-alloy powder at a temperature ranging 300 to 500xc2x0 C. and for a period ranging from 24 hours to 48 hours.
Preferably, the amalgamatable dental alloy powder of the present invention is prepared by the process comprising steps I) and II).
Preferably, said Agxe2x80x94Cuxe2x80x94Sn powder with a majority thereof have a particle size less than 20 microns and said Agxe2x80x94Cuxe2x80x94Pd powder with a majority thereof have a particle size less than 35 microns. More preferably, said Agxe2x80x94Cuxe2x80x94Sn powder with a majority thereof have a particle size less than 10 microns and said Agxe2x80x94Cuxe2x80x94Pd powder with a majority thereof have a particle size less than 25 microns.
Preferably, said heat treated Agxe2x80x94Cuxe2x80x94Sn powder and said heat treated Agxe2x80x94Cuxe2x80x94Pd powder are separately subjected to said pickling treatment.
Preferably, said second heat treatment comprises heating said Agxe2x80x94Cuxe2x80x94Sn powder at a temperature ranging from 300 to 500xc2x0 C. for a period ranging from 24 hours to 48 hours, and said third heat treatment comprises heating Agxe2x80x94Cuxe2x80x94Pd powder at a temperature ranging from 100 to 500xc2x0 C. for a period ranging from 24 hours to 48 hours. More preferably, said second heat treatment comprises heating said Agxe2x80x94Cuxe2x80x94Sn powder at 400xc2x0 C. for 24 hours, and said third heat treatment comprises heating Agxe2x80x94Cuxe2x80x94Pd powder at a temperature ranging from 100 to 400xc2x0 C. for 48 hours.
When the amalgamatable dental alloy powder of the present invention is prepared by the process comprising steps I) and II), the composition thereof preferably comprises 60-70 wt % Ag; 10-20 wt % Cu, 10-30 wt % Sn and 2-4 wt % Pd, and more preferably 67.2 wt % Ag, 12.0 wt % Cu, 17.4 wt % Sn, and 3.4 wt % Pd.
Preferably, said Agxe2x80x94Cuxe2x80x94Sn powder comprise 70 wt % Ag, 4 wt % Cu and 26 wt % Sn, and said Agxe2x80x94Cuxe2x80x94Pd powder comprise 62 wt % Ag, 28 wt % Cu and 10 wt % Pd.
In the preferred embodiments of the present invention, two different types of amalgamatable dental alloy powders were made and used in the preparation of amalgams, which include a single-alloy powder, and a mixed powder of a matrix alloy and a dispersant alloy. Further, a commercially available amalgamatable single-alloy powder was directly used. As shown by the following examples, the amalgamatable dental alloy powders prepared according to the present invention have significantly lower early stage mercury vapor release rates and comparable mechanical properties in comparison with the commercial amalgam dental alloy powders.